1. Field of the Invention
This invention relates to a method for manufacturing a steel sheet which is electroplated on only one side thereof, hereafter referred to as a one-sided electroplated steel sheet. More particularly, it relates to a manufacturing method for one sided electroplated steel sheet having a zinc-based plated coating on one side, and an unplated side having good appearance and adaptability to chemical conversion treatment. The resulting steel sheet is suitable as rust-resisting steel sheet for use in automobile manufacture.
2. Prior Art
Recently, one-sided electroplated steel sheets which are usually coated with a corrosion-resistant zinc-based electroplating on one side have been used extensively as rust-preventing steel sheet in the manufacture of automobile bodies in order to improve the durability of automobiles. The other side of such a one-sided electroplated steel sheet which is not coated with plating will be referred to as the unplated side.
The term "zinc-based plating" used herein encompasses both pure zinc plating and zinc alloy plating. Of the zinc-based one-sided plated steel sheets, the proportion of those plated with a zinc alloy such as Zn--Ni is increasing since they have excellent corrosion resistance.
An automobile body can be made from a one-sided plated steel sheet with the plated side facing the inner surface of the body. In this case, the inner surface of the body which is not coated with paint has good corrosion resistance due to the zinc-based plating. Since the outer, painted surface of the body is formed from the unplated side of the steel sheet, it exhibits good weldability and adhesion to paint, properties inherent to unplated surfaces of steel sheets.
One-sided electroplated steel sheets are manufactured by passing a steel sheet through a plating bath while passing a current between the steel sheet which serves as a cathode and an anode on either side of the steel sheet. This manufacturing method involves the following problem.
In the continuous manufacture of a zinc-based one-sided plated steel sheet, an acidic plating solution such as a sulfate solution or a chloride solution is employed. The acidic plating solution attacks and corrodes the unplated side of the steel sheet, thereby forming black smudges caused by deposition of corrosion products. This phenomemon is called acid burning and it deteriorates not only the appearance of the unplated side by the discoloration, but also the adaptability to chemical conversion treatment such as phosphating which must be performed prior to painting to improve the adhesion of a paint. As a result, the adhesion of paint to such an acid-burned surface is deteriorated.
Therefore, in actual plating operation, mechanical or electrolytic polishing is performed on the unplated side of the one-sided plated steel sheet in order to remove the metallic or other smudges deposited on the unplated side.
Mechanical polishing, which is performed by brushing or other abrasive means can remove the black smudges on the unplated side to a certain degree, but it is accompanied by abrasion of the underlying steel plate. This leads to a decrease in the surface roughness of the unplated side, which may cause slip to occur in the blanking line when the plated steel sheet is blanked out during automobile manufacture.
For this reason, the black smudges formed on the unplated side are usually removed by electrolytical treatment after one-sided electroplating. The following methods have been proposed for carrying out the post-plating electrolytic cleaning of the unplated side.
(a) Electrolysis is performed in a solution containing 50-300 g/l of a mixture of a sulfate and a phosphate at a pH of 5-9 [Japanese Unexamined Patent Application Publication No. 62-99494(1987)];
(b) Electrolysis is performed in a bath containing a sulfur compound by a combination of anodic treatment and cathodic treatment [Japanese Unexamined Patent Application Publication No. 62-13595(1987)];
(c) After thin plating, electrolysis is performed in a bath containing a pH buffer and an oxidizing agent by passing a current indirectly between the unplated side, which serves as an anode, and the plated side, which serves as a cathode [Japanese Unexamined Patent Application Publication No. 61-163292(1986)];
(d) Electrolysis is performed in an aqueous solution of a water-soluble sulfate which contains triethanolamine [Japanese Unexamined Patent Application Publication No. 61-117300(1986)];
(e) Electrolysis is performed in an aqueous solution containing a sulphate or a phosphate by anodic treatment [Japanese Unexamined Patent Application Publication No. 61-106800(1986)];
(f) Electrolysis is performed in a conductive bath at pH 4-10 containing 0.05-2.0% of a surfactant by anodic treatment [Japanese Patent Publication No. 61-36597(1986)];
(g) Electrolysis is performed in a solution containing a particular kind of sulfur compound by anodic treatment or cathodic treatment [Japanese Patent Publication No. 61-41990(1986)];
U.S. Pat. No. 4,464,232 also discloses post-plating electrolytic polishing of a one-sided electroplated steel sheet.
These methods of post-plating electrolysis require several electrolytic cells in order to achieve a satisfactory cleaning effect. However, due to space and cost restrictions, only one or two electrolytic cells are usually employed and it is difficult to sufficiently clean the unplated side in the short period of several seconds during which the steel sheet is passed through the cells. Particularly in the manufacture of a one-sided plated steel sheet with a zinc alloy (such as Zn--Ni alloy) coating, the alloying element such as Ni which is nobler than Fe is inevitably deposited on the unplated side during electroplating. It is more difficult to remove such nobler alloying elements deposited on the unplated side by post-plating electrolysis carried out for only a short period.